1) Field of the Invention
This invention relates to a wavelength multiplexing transmission apparatus and a wavelength demultiplexing reception apparatus suitable for use with wavelength multiplex transmission of a high density.
2) Description of the Related Art
In recent years, a demand for increase in capacity of a transmission apparatus has been and is progressively increasing as the information society advances progressively. Particularly, increase of the speed in electronics (an increase in transmission speed of an electric signal) is getting difficult after the speed exceeds 10 Gb/s, and it is anticipated significantly to achieve an increase in capacity by an increase in speed on an optical signal level, particularly, by signal transmission to which a wavelength multiplex system is applied.
As a wavelength multiplex system, narrow-band wavelength multiplexing in the 1.55 μm band which provides a wavelength with which the loss of an optical fiber is lowest is considered promising and has already been put into practical use. Further, since this wavelength band coincides with an application wavelength band of an optical amplifier for which an optical fiber doped with erbium is used and also the loss of a wave combiner/separator provided for performing wavelength multiplexing or wavelength demultiplexing can be compensated for by the optical amplifier, wavelength multiplexing of such a large number of waves that is not conventionally available can be performed.
However, in a transmission apparatus which adopts such a wavelength multiplex system as described above, when wavelength arrangement of the wavelengths of a plurality of transmission optical signals (transmission lights) is performed in a narrow band and in a high density in the 1.55 μm band, small variations occur with the wavelengths of the transmission light signals by various causes, and also where a small wavelength variation occurs with some transmission light in this manner, the signal of the channel may not be transmitted successfully or may have an influence on the optical signal of a wavelength of an adjacent channel and bring about crosstalk with the adjacent channel.
In such an instance, a communication error may possibly occur not only with the channel with which the wavelength variation occurs but also with the channel with which the crosstalk occurs.
Accordingly, a transmission apparatus which performs high density wavelength multiplexing or wavelength demultiplexing described above not only has a subject to be solved in that wavelengths of transmission light sources must be stabilized, but also has another subject in that, should wavelength displacement occur, crosstalk with another channel must be prevented.
Further, while some optical signal transmission apparatus include a temperature stabilization circuit or a like circuit for a light source in order to secure stability of the temperature, where the stability of a thermistor which monitors the environment temperature of the light source is not sufficiently high, there is a subject in that it must be taken into consideration that a variation may occur because of deterioration of the thermistor as time passes. On the other hand, also there is another subject in that care must be taken of deterioration of a transmission light source itself which outputs an optical wavelength as time passes.